The projection-type image display device is widely used for purposes ranging from a personal theater to business presentations. An example of the projection-type image display device is disclosed in JP 49007436 B2 (hereinafter referred to as “Patent Literature 1”).
The projection-type image display device disclosed in Patent Literature 1 will be described with reference to FIGS. 1A to 1D.
FIG. 1A is a plan view schematically showing the interior of a projection-type image display device disclosed in Patent Literature 1. As shown in FIG. 1A, projection-type image display device 1 disclosed in Patent Literature 1 includes light source unit 2 configured to emit light, and image projection unit 3 configured to form an image, by using the light emitted from light source unit 2, and to project the image. Light source unit 2 and image projection unit 3 are attached to base member 4.
FIG. 1B is an enlarged plan view of light source unit 2 shown in FIG. 1A. As shown in FIG. 1B, light source unit 2 includes laser light source 5 configured to emit blue laser beam, and fluorescent wheel unit 6 configured to convert the blue laser beam emitted from laser light source 5 into red light and green light. FIG. 1C is a front view of fluorescent wheel unit 6 shown in FIG. 1B, and FIG. 1D is a schematic sectional view of fluorescent wheel unit 6 shown in FIG. 1B.
As shown in FIGS. 1B, 1C, and 1D, fluorescent wheel unit 6 includes fluorescent wheel 10 in which phosphors 8 and 9 are applied on circular substrate 7, and motor 11 for rotating fluorescent wheel 10. Red light is emitted from phosphor 8 when phosphor 8 is irradiated with a laser beam, and green light is emitted from phosphor 9 when phosphor 9 is irradiated with a laser beam. Fluorescent wheel 10 includes transmission region 12 for transmitting the laser beam.
By irradiating phosphors 8 and 9 and transmission region 12 of rotating fluorescent wheel 10 with the laser beam emitted from laser light source 5, red and green fluorescent lights and a blue laser beam are obtained. Image projection unit 3 (see FIG. 1A) forms an image by using the red, green, and blue lights obtained by means of fluorescent wheel unit 6, and projects the image.
JP 2010-86815 A (hereinafter referred to as “Patent Literature 2”) discloses a projection-type image display device including a light source unit different from light source unit 2 disclosed in Patent Literature 1. The projection-type image display device disclosed in Patent Literature 2 will be described with reference to FIGS. 2A and 2B. Components identical to those shown in FIG. 1A are denoted by identical reference numerals, and description thereof will be omitted.
FIG. 2A is a plan view schematically showing the interior of the projection-type image display device disclosed in Patent Literature 2. As shown in FIG. 2A, projection-type image display device 13 disclosed in Patent Literature 2 includes light source unit 14 and image projection unit 3 that are attached to base member 4. FIG. 2B is an enlarged plan view of light source unit 14 shown in FIG. 2A.
As shown in FIG. 2B, light source unit 14 includes laser light sources 15a, 15b, and 15c, and fluorescent wheel units 16a, 16b, and 16c. 
Fluorescent wheel unit 16a includes fluorescent wheel 19a in which phosphor 17a, that emits blue light when phosphor 17a is irradiated with a laser beam, is applied on circular substrate 18a, and motor 20a for rotating fluorescent wheel 19a. By irradiating phosphor 17a of rotating fluorescent wheel 19a with the laser beam emitted from laser light source 15a, blue light is obtained.
Similarly, fluorescent wheel unit 16b includes fluorescent wheel 19b in which phosphor 17b, that emits green light when phosphor 17b is irradiated with a laser beam, is applied on circular substrate 18b, and motor 20b for rotating fluorescent wheel 19b. Fluorescent wheel unit 16c includes fluorescent wheel 19c in which phosphor 17c, that emits red light when phosphor 17c is irradiated with a laser beam, is applied on circular substrate 18c, and motor 20c for rotating fluorescent wheel 19c. By irradiating phosphors 17b and 17c of rotating fluorescent wheels 19b and 19 with the laser beams emitted from laser light sources 15b and 15c, green and blue lights are obtained.
Light source unit 14 further includes dichroic mirror 21a for reflecting the green light and transmitting the blue light, and dichroic mirror 21b for reflecting the red light and transmitting the blue and green lights.
Dichroic mirror 21a is disposed at a position where the blue light obtained by means of fluorescent wheel unit 16a and the green light obtained by means of fluorescent wheel unit 16b intersect each other. The green light from fluorescent wheel unit 16b is reflected by dichroic mirror 21a, and the traveling direction of the green light is changed into the traveling direction of the blue light from fluorescent wheel unit 16a. 
Dichroic mirror 21b is disposed at a position where the blue light obtained by means of fluorescent wheel unit 16a and the red light obtained by means of fluorescent wheel unit 16c intersect each other. The red light from fluorescent wheel unit 16c is reflected by dichroic mirror 21b, and the traveling direction of the red light is changed into the traveling direction of the blue light from fluorescent wheel unit 16a. 
Image projection unit 3 (see FIG. 2A) forms an image, by using the blue, green, and red lights that are obtained by means of fluorescent wheel units 16a, 16b, and 16c, and projects the image.
A reason for rotating fluorescent wheels 10, 19a, 19b, and 19c shown in FIGS. 1B and 2B is to prevent a reduction in fluorescence conversion efficiency and thermal damage of phosphors 8, 9, 17a, 17b, and 17c. 
For example, when a part of phosphor 17a is irradiated with the laser beam from laser light source 15a for a long period of time, the temperature of the part rises. When the temperature exceeds a predetermined value, a phenomenon that is known as thermal saturation occurs in which the amount of fluorescent light, that is emitted from phosphor 17a, is reduced. When the temperature of that part of phosphor 17a further rises, the part may be burned. By rotating fluorescent wheel 19a, the energy of the laser beam is distributed over the entire area of phosphor 17a, thereby preventing a partial reduction in fluorescence conversion efficiency and preventing phosphor 17a from burning.
The temperature at which thermal saturation occurs varies among phosphor 17a that emits the blue light, phosphor 17b that emits the green light, and phosphor 17c that emits the red light. JP 2011-145681 A (hereinafter referred to as “Patent Literature 3”) discloses a projection-type image display device designed to control the numbers of rotations of fluorescent wheels 19a, 19b, and 19c according to the thermal saturation characteristics of phosphors 17a, 17b, and 17c. 
Recently, a light source unit has been proposed that includes a fluorescent wheel unit configured to convert a laser beam into yellow light, and a color wheel unit configured to divide the yellow light into red light, blue light, and yellow light. The color wheel unit includes a color wheel in which a plurality of color filters is arrayed on a circular substrate to transmit predetermined color light, and a motor for rotating the color wheel.
By irradiating the rotating color wheel with yellow light, red, green, blue, and yellow lights are obtained. A phosphor that emits yellow light has high fluorescence conversion efficiency and excellent thermal saturation characteristics. The use of the phosphor that emits yellow light allows the luminance of the light source unit to be further increased.